If the metal frame of a building is bonded to the metal water pipe as allowed in 250.52(A)(2) method #2, then the metal frame is allowed to be the supplemental electrode as allowed by 250.53(D)(2)
With this scenario though, when the metal water pipe is replaced with plastic down the road, the electrode system is lost altogether.
We need to supplement this scenario with a additional electrode such as ground rods because the metal frame of the building will no longer be effectively grounded when the metal water pipe is replaced with non-metalic pipe.
shortcircuit
